Adding a power-management MCU to your system reduces standby energy use -

Adding a power-management MCU to your system reduces standby energy use

This “Product How-To” article focuses how to use a certain product in an embedded system and is written by a company representative.

The number of electrical products used in private homes and offices isgrowing at an extremely fast pace. The problem with most of theseproducts, such as A/V products and office equipment, is that theyconsume considerable amounts of electrical power during standby. Thisis a waste of electrical power and money, and has a negative impact onthe environment.

That is why today, for a large number of products, there areregulations that fix stringent limits for standby energy consumption.This article proposes a power management solution for electricaldevices during standby mode, compliant with power efficiencyspecifications.

Besides intelligent power management, an advantage of the proposedsystem architecture is its general purpose usage independent of theproduct. It can be easily adapted and embedded in consumer devices,office equipment and home appliances.

Normal standby
Standby mode refers to a product's state when it is switched off butstill connected to the main power supply. 

Active mode refers to a product's state when it is switched on andrunning normal functions. Because many devices stay in standby modemost of the time – a DTV or a printer, for example – an energy cutbackduring this phase would determine an overall energy savings.

Current specifications from Energy Star require standby powerconsumption be less than or equal to 1-2W, depending on the type ofequipment. For A/V products this value is fixed at 1W.

Power management solution
ST proposes a smart power management control, necessary to designsystems with sustainable principles. This is done by adding a generalpurpose low-power MCU to the main system that has specific firmwareresponsible for power control.

Figure 1 below shows theblock diagram of a generic system that could be of an A/V product,office equipment, or home appliance product. Independent from theapplication, we can distinguish two parts – the main system block andthe power management block.

Figure1: The general system block diagram is shown

The main system block performs the primary function of the system;its hardware topology is different for different products, but each onehas a main IC controller with an I2C bus and at least one wake-upsource.

The main IC controller manages each system function, interfacingwith a standby controller, wake-up sources and the other devices.

Wake-up sources are events that can command the standby controllerto exit the standby mode and return to the active mode. Common wake-upsources are a remote control receiver, a keyboard, VGA and HDMI inputs.

A wake-up source could be also a timer event, used to record, starta special operation or for periodic update. During active mode, thewakeup sources interface with the main IC controller to perform thefunctions related the command.

The power management block is a general purpose block independentfrom the main system. The brain of this subsystem is an 8-bit low-powerMCU typically used as standby controller. It receives inputs from themain IC controller and wake-up sources, switching on and off, throughthe relay, the full on power supply block that supplies most of themain system.

During standby mode, the low power MCU and the wake-up sources arepowered by the auxiliary power supply. According to this architecture,the system works in two different modes (Figure 2 below ):

The active mode, when the main IC controller and other devices arepowered.

The standby mode, when only the low-power MCU and the wake-upsources are powered. The default state after reset is the active mode.

Figure2: The proposed architecture has two modes.

Active mode
As soon as the system is plugged into the mains power, each part of thesystem is powered and the system is running.

The main IC controller will manage each function; it detects userinputs from wake-up sources such as remote control and front panels; itinterfaces with other devices and the low-power MCU using the I2C bus,where the main IC controller is configured as master and the otherdevices, low-power MCU included, are configured as slaves.

In the communication between the main IC controller and low-powerMCU during active mode, the main IC controller updates low-power MCUregisters, to program, for instance, new timer events, and to send thestandby command request.

Once the standby controller receives the power off command from themain IC controller, it enters standby mode.

Standby mode
When the low-power MCU receives the standby command from the main ICcontroller, it puts the system in standby mode, switching off the relayconnected to the full on power supply.

At standby mode, only the low power MCU and the wake-up sources arepowered on. During this phase, the low-power MCU enters halt mode,waiting for an external input or timer event.

Once one of these events happens, the low-power MCU exits from haltmode, decodes the signal received and switches on the whole systemthrough the relay.

The kind and function of wake-up event is communicated through theI2C bus to the main IC controller to perform the related operation; nowthe system is in active mode, and each normal function runs.

Figure3: The power management block shows that the wake-up sources are an IRreceiver for a remote control signal and a front panel button.

Here's an example of how to implement standby power management controlfor a consumer application using STMicroelectronic's ultralowcostST7FOXA0 8bit MCU.

The power management Block (Figure3 above ) can be adapted to different main system blocks. Thewake-up sources of the system are an IR receiver for remote controlsignals and a front panel power on/off button.

The 8bit MCU and the wakeup sources are powered at 3.3V; they arethe permanent live part of the system. The main IC controller, on theother hand, is powered by a main power supply that is switched on andoff by the 8bit MCU according to the standby command.

The MCU interfaces with the main IC controller (present in the mainsystem) through the I2C communication where the 8bit MCU is configuredas slave and the main IC controller as master. The main MCU featuresare:

1) Emulated low speed I2Cslave interface communication;
2) Remote controller receiverand decoder wake-up event;
3) Front panel wake-up event;and,
4) Main power supply commandwith programmable polarity.

The I2C slave interface is emulated just using two I/O pins. Theremote control receiver wakes up the system when in standby mode. Thedecoding routine is implemented using an I/O pin configured as anexternal interrupt and the 12bit auto reload timer peripheral.

The front panel wakes up the system when in standby. In this system,a power ON/OFF push button is connected to a pin configured as externalinterrupt.

The main power supply is driven thought a relay using an I/O pinconfigured as output. To switch the relay on and off the outputpolarity can be programmed accordingly. During standby mode, the MCU isplaced in halt mode to further reduce power consumption; it will exitfrom standby mode using the external interrupt.

Figure4: During standby mode, the MCU is placed in halt mode to furtherreduce power consumption.

The application flow is shown in Figure4 above . To contribute significantly to the reduction of energyconsumption, a smart power solution for products during standby hasbeen proposed.

The system architecture is based on a low-power MCU, which is ableto greatly reduce the energy consumption during standby mode bydisconnecting most of the system from the main power. This saves energyand reduces the impact of the device on the environment.

Alessandra Di Pietro is anApplication Engineer at STMicroelectronicsN.V.

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